The function of selenium in thiolase isolated from Clostridium kluyveri is being studied. Preliminary isoelectric focusing studies have revealed that the selenothiolase is a basic protein (pI 8.7-9.0) and this finding has led to a modification of the isolation procedure. Previously, the final purification step had been chromatography on Matrex Gel Green resulting in a two-fold increase in specific activity but with a 50% loss of total activity. Chromatography on carboxymethyl-Sephadex proved to be a more satisfactory step in that it gave a similar increase in specific activity with 96% recovery of the total activity. Since the selenium moiety of thiolase is selenomethionine, antibodies specific for selenomethionine might be useful as a structural probe, a means of rapid isolation of the enzyme, or in surveys from other organisms, etc. Accordingly, selenomethionine has been covalently coupled to bovine serum albumin, in order that sheep antibodies may be raised.